Current trends in catalysis research include the gradual replacement of homogeneous catalysts by heterogeneous catalysts. The advantages of using a solid base catalyst instead of a liquid base in catalytic reactions are:
(1) The high catalytic activity and selectivity and high purity of product;
(2) The catalysts can be easily separated;
(3) Preventing corrosion of equipment, reduction in effluent and more environmentally friendly.
However, heterogeneous catalysis systems usually have several shortcomings such as smaller reaction interface area and bigger transmission resistance, etc. These shortcomings significantly reduce the catalytic activity compared with homogeneous catalysis systems. There are also other reasons which limit the extensive application of heterogeneous catalysis systems.
Layered double hydroxides (LDHs) are a promising prospect in the field of heterogeneous catalysis. Shape-selective catalytic properties can be introduced by controlling the particle size and distribution and by adjusting the porosity. Nano-scale mixed metal oxides that have small particle size and large specific surface area have very efficient contact with the substrate, and give rise to high catalytic activities and excellent thermal stability. The disadvantage of these catalytic systems is the poor dispersion of the liquid-solid system, as well as difficulties in separation and reclamation of the catalyst. Furthermore, the large bed resistance caused by the small size of the catalyst particles in gas-solid catalysis systems leads to difficulties in their utilization in industry.
EP0421677A1 and EP0421678A1 relate to alkaline LDHs and calcined LDHs as solid base catalysts in glycol ether synthesis. LDHs can be represented by the general formula [M2+aM3+b(OH)9(2a+3b)][X]b, where M2+ and M3+ are divalent and trivalent metal cations, respectively; X is interlayer anion. LDHs were calcined to give Mg—Al—O compounds. Both of these works have not described how to solve the problems of dispersal, separation and reclaim of the catalyst however.